3D printing microfluidic chips
A desktop resin printer makes working microfluidic chips. Not at the scale of photolithography, but at the scale most mixing, gradient and droplet work needs: channels from about 0.1 mm wide on a good machine, and 0.2–0.3 mm on almost any.
OpenChip designs the chip and exports the STL. This section of the wiki covers everything after that. This page is the short version; each part links the page that covers it properly.
What channel size can a desktop printer make?
Section titled “What channel size can a desktop printer make?”It depends on the pixel pitch: the size of one pixel on the printer’s LCD, which sets the finest thing the machine can draw in plan.
| Printer class | Pixel pitch | Realistic minimum channel |
|---|---|---|
| 2K / 4K, small | 35–50 µm | 0.2–0.3 mm |
| 8K / 12K | 17–22 µm | 0.1–0.15 mm |
| Large-format | 45–55 µm | 0.25–0.35 mm |
A channel needs five to six pixels of width to survive washing and curing. One pixel prints as nothing, three prints unreliably. That rule is where the shipped printer profiles get their minimum feature size: 0.12 mm for a 20 µm machine, 0.2 mm for a 35 µm one.
So a 150 µm channel is reliable on an 8K-class printer and out of reach of a 35 µm one. Above 0.3 mm, almost anything works.
Expect the printed channel to come out narrower than drawn, typically by 0.02–0.08 mm per side. It is repeatable for a given resin, so you measure it once and design with it.
See Resolution vs. channel size for why, and for practical floors on walls, channel heights and port bores.
What to buy
Section titled “What to buy”Any current mSLA (masked stereolithography) printer will do. The differences between machines matter less than the resin you put in one.
- Printer. 4K or 8K. 8K if you need channels below about 0.15 mm.
- Resin. A grey standard resin, low viscosity. Pigmented resins cure less deeply past the exposed layer, so small channels stay open; clear resins close them up.
- Cleaning. Isopropanol (91 % or better) and a wash station, or a water-washable resin.
- Curing. A UV cure box. Not sunlight, which is slow and uneven.
If cells are involved, test the resin first. Cured photopolymer leaches unreacted monomer and photoinitiator.
See Choosing a printer and resin.
How the channels get sealed
Section titled “How the channels get sealed”The decision that matters most, made on the chip parameters panel before you print.
- Two-piece, the default. The chip splits into a channel plate and a flat lid, bonded after printing. The channels are open grooves until then, so uncured resin rinses straight out. Below about 0.5 mm it is close to the only realistic option.
- Monolithic. One sealed block with the channels as voids inside it. Nothing to bond and nothing to leak, but the resin has to come out through the ports.
- Glass slide. An open-faced chip with a pocket for a microscope slide, which becomes the lid. For anything you want to see into.
From STL to working chip
Section titled “From STL to working chip”- Print a test coupon first. An hour that measures your real minimum channel, your real minimum wall, and whether your ports fit your fittings. See A first test-print protocol.
- Print flat, channel side up, no supports. Channels stay defined by pixels and ports stay round. See Orientation and supports.
- Get the resin out before it sets. Drain by gravity, flush with a syringe, and check each channel against a light. Resin left inside and then cured is permanent. See Draining uncured resin.
- Wash, then cure, in that order. Under ten minutes in isopropanol in total, then 5–15 minutes in a UV box. See Washing and post-curing.
- Bond the lid on a two-piece chip. A very thin film of the same resin, on the lid rather than the channel plate, so it cannot run into a groove. See Bonding and sealing.
- Connect and prime. A luer fitting or a tubing socket, strain relief on the line, and connect wet to wet so no bubble gets in. See Tubing and fittings.
Where first chips go wrong
Section titled “Where first chips go wrong”Three places, in rough order of how often:
- Resin cured inside a channel. Not drained before the post-cure. The channel is gone.
- Two channels merged. The wall between them cured through. The default minimum printable wall is 0.4 mm, and it is the number people are most often optimistic about.
- A port that does not fit. Printed sockets come out slightly off size, and the test coupon tells you by how much.
OpenChip’s design checks warn about each of these before you print. None of them blocks an export. See the design-check catalogue.